As the method for producing the compound oxide powder, there are generally known the simultaneous powder sintering method, the co-precipitation method and the alkoxide method. In the simultaneous powder sintering method, the powder of a precursor such as an oxide, a carbonate or a hydroxide of each metal is mixed and sintered. In the co-precipitation method, a colloidal dispersion liquid of an oxide or hydroxide is produced by adding alkali to an aqueous solution of a plurality of inorganic metallic salts thereby to neutralize it. In the alkoxide method, a plurality of metallic alkoxides dissolved in an organic solvent are hydrolyzed by adding water to them.
The simultaneous powder sintering method is limited in the fineness of the powder, and a sintering operation at a high temperature is needed for obtaining a compound oxide from the powder. In the hot sintering operation, the granules grow, and their surface area drops. As a matter of fact, it is difficult to obtain the fine powder of the compound oxide which has a high surface area and which is completely homogenized at the atomic level.
The co-precipitation method utilizes the neutral sedimentation reactions of inorganic ions in an aqueous solution. Although the colloidal particles to be produced have fine diameters, the sedimentation reactions of the inorganic ions depend on the pH. Therefore, the individual colloidal particles have a tendency that they become individual particles of metal oxides or metal hydroxides, but do not produce the compound oxides which are not homogeneously mixed at the atomic level.
The existing alkoxide method utilizes the hydrolyses of a plurality of metallic alkoxides in the organic solvent. However, the stability and the hydrolytic rate are different according to the kind of the metallic alkoxide. Therefore, the priority for producing the oxides is between the metals, and the method has also failed to produce the compound oxides which is homogeneously mixed at the atomic level.
In JP-A-10-216517 or JP-A-7-246343, on the other hand, there is disclosed a method, in which catalytic active particles (i.e., compound oxide particles such as precious metal particles or ceria-zirconia particles) are synthesized, and in which oxide carriers are then simultaneously synthesized around the catalytic active particles in that reaction field.
However, the aforementioned invention, as described in JP-A-10-216517 or JP-A-7-246343, is intended to suppress the sintering by the particle migrations of the catalytic active particles thereby to suppress the thermal deterioration of the catalyst, but not to improve the dispersion of the metal ions constituting the compound oxides.